A battery-driven electronic apparatus, such as a mobile phone, a tablet terminal, a notebook personal computer, a portable audio player, a digital camera and so on, is equipped with a secondary battery such as a lithium ion battery or the like and electronic circuits including a microcomputer, a LCD panel, an audio output circuit, a radio communication circuit and so on. Here, a power supply voltage to be supplied to each electronic circuit is different for each part. In addition, in order to supply a proper power supply voltage to each electronic circuit, a power supply circuit to step down or step up a battery voltage is equipped in the electronic apparatus.
FIGS. 1A and 1B are circuit diagrams showing an example of a power supply circuit equipped in an electronic apparatus. A power supply circuit 400 of FIG. 1A is a step-down DC/DC converter. The power supply circuit 400 steps doer an input voltage VIN (=VBAT) from a battery 2 and supplies a power supply voltage VOUT to an electronic circuit (load 4) connected to an output line 402.
A consumption current of an electronic circuit contained in the electronic apparatus, i.e., a load current IOUT of the DC/DC converter 406, is varied in a large dynamic range depending on a state of the electronic apparatus. More specifically, the consumption current decreases to about zero in a standby mode of the electronic apparatus and increases from several hundred mA to several A in an active mode thereof.
In general, efficiency of a DC/DC converter decreases in a light load state with a small load current. This is because, even when the load current IOUT, i.e., power supplied to the load 4, is decreased, power required to switch switching elements M1 and M2 (switching loss) is not decreased as much. In order to increase the efficiency in the light load state, there has been proposed a PFM (pulse frequency modulation) mode to decrease a switching frequency. However, even in the PEM mode, high efficiency is limited since the switching loss is not zeroed.
A power supply circuit 400a of FIG. 1B includes a linear regulator 404 in addition to the DC/DC converter 406. In the power supply circuit 400a of FIG. 1B, each of the linear regulator 404 and the DC/DC converter 406 switches between an active state and an inactive state independently. Depending on the load current IOUT, by activating the DC/DC converter 406 in a range of heavy load state to light load state and activating the linear regulator 404 in a state with a smaller load current (referred to as a super light load state in the specification), high efficiency can be obtained in a wide range of the load current IOUT. 
However, in the power supply circuit 400a of FIG. 1B, the DC/DC converter 406 and the linear regulator 404 have their respective separate feedback loops. Therefore, when the DC/DC converter 406 and the linear regulator 404 are switched, feedback is interrupted, which results in output voltage variations such as overshoot, undershoot, ripple and so on, and thus it takes a long time to stabilize the output voltage.
In addition, since only the load 4 of one channel is driven, in order to supply different power supply voltages to various loads, there is a need to provide the power supply circuit 400a for each of the loads, which results in an increase in cost and circuit area.